Multilateration is a process by which an object's location can be fixed from a radio transmission made from the object and received at a number of receiver station stations. The time of arrival at the receiver stations is noted and passed to a central processing unit where, with knowledge of the position of the receiver stations, the position of the object is determined. Alternatively, the signals are passed over a link to the central processing unit which determines the time of arrival at the receiver station with reference to the delay caused by the link. A process of signal correlation is carried out in the central controller in which the received signals are compared with a copy of the transmitted signal.
Multilateration systems are used, in particular, to provide the position of aircraft in flight or on airport runways. A signal transmitted by a transmitter on the aircraft is received by a number of receiver stations at known locations on the ground. The signal is transmitted by a 1090 MHz Secondary Surveillance Radar (SSR) transponder and is one of a number of known code types or formats such as Mode A/C and Mode S. By measuring the time of arrival of the signal at each of the receiver stations (or deriving it as mentioned above), and with the knowledge of their locations, it is possible to calculate the position of the aircraft at the time of transmission. Such a system and a multilateration technique is described in patent GB2250154, the contents of which are incorporated herein by reference.